Various methods for the thermal reaction of carbonaceous fuels for powering conventional engines and power plants are known. One system which has been used in connection with stationary gas turbines is a catalytic combustor system. These systems are based on a mechanism which has been called catalytically supported thermal combustion and the physical mechanisms that allow sustained catalytic combustion to occur at high reaction rates are discussed in detail in Pfefferle, U.S. Pat. No. 3,928,961.
Basically, the catalytically supported thermal combustion state is achieved when there is a sufficiently intense catalytic combustion adjacent to the walls of the chamber containing the catalyst to maintain a high bulk temperature and to thereby support thermal combustion in the free stream of the fuel-air admixture. If the temperature is not sufficiently high, thermal combustion will be incomplete and substantial quantities of the fuel will not be burned. Accordingly, the two design parameters for steady state operations of such a catalytic reactor are the mixture inlet temperature and the fuel-air ratio. The latter parameter controls the temperature rise in the reactor.
Tests which have been performed for the United States Environmental Protection Agency have identified the minimum preheating temperatures that are required for lean catalytic operations. When the fuel is natural gas, values of about 350.degree. F. for noble metal catalysts and 300.degree. F. for base metal oxide catalysts are typical. For liquid fuels such as No. 2 heating oil, values in the range of 300.degree.-600.degree. F. have been demonstrated. In typical gas turbine systems, the compressor discharge temperatures are in the neighborhood of 600.degree. F. and therefore no additional preheating of the incoming air stream is necessary.
In general, the maximum operating temperature of a catalytic combustor is limited by material capabilities to below 3000.degree. F. and the minimum temperature is generally limited by the combustion stability of the fuel-air system to above 1800.degree. F. Since the current commercial stationary engines have turbine inlet temperatures of approximately 1850.degree.-2000.degree. F., they are well suited for use with catalytic combustors. During the steady state operation under load, the catalyst temperature is somewhat in excess of the turbine inlet temperature because of liner cooling and the dilution air which is added to the reactor combustion products. Operation at about 2300.degree.-2400.degree. F., in fact, provides a substantial degree of flexibility in the selection of appropriate catalyst systems. However, since inlet temperatures are well below 1800.degree. F. in combustors during the start up sequence and during the loading sequence, and since air scheduling is not considered to be a desirable control technique, alternate means of start up are required.
It will be appreciated that the peak temperatures in the catalytic bed equals the adiabatic flame temperature of the entering fuel-air mixture. Accordingly, if the operating temperature is to be 2300.degree.-2400.degree. F., the maximum bed fuel-air ratio is limited.
The most efficient and stable combustion occurs in a catalytic reactor when the burning mixture is in contact with the catalyst for a sufficiently long period. When the contract period is too short, insufficient energy is generated adjacent to the catalyst surface to sustain combustion in the main or free stream. While a number of analytical models of this complex have been developed, two parameters in particular--face velocity and nominal residence time of the mixture in the reactor--have been used to evaluate experimental performance in actual use.
The maximum values of face velocity, i.e., the mean velocity of the mixture upstream of the catalyst, range from about 80-150 ft./sec. for various catalysts. This condition also requires that the catalytic reactor have a minimum frontal area for the combustor air-fuel flow to traverse.
The maximum acceptable face velocity is established by the given values of the catalyst operating temperature, pressure, stoichiometry and preheating temperature. Further increases in velocity result in blowouts. Lower limits on the velocity of the mixture are set by the mixture flame speeds in order to avoid flash-back and are determined by proper selection of the combustor cross-sectional area. The maximum achievable velocity depends on flow conditions and catalyst parameters such as type, monolith cell size, and web thickness. Use of noble metal catalysts with face velocities in excess of 100 ft./sec. and metal oxide catalysts in excess of 150 ft./sec. have been demonstrated.
The minimum residence time in the combustor is a function both of the face velocity and the reactor length. Since the velocity increases as the fuel burns and the temperature rises, the face velocity establishes a minimum velocity level just upstream of the catalyst bed.
The foregoing description has been concerned with a catalytic combustor operating at or near its design point. However, gas turbine operations require a much broader range of inlet temperatures and fuel-air ratios. In a typical situation, the fuel-air ratio limits are approximately 0.005-0.025 and the inlet temperature ranges from ambient at ignition to about 600.degree. F. on the simple cycle machines. Catalyst ignition does not readily occur, however, at inlet temperatures which are less than about 1000.degree. F. for most catalysts. In order to overcome this problem, the most common solution is to employ a pilot burner for ignition and acceleration and by waiting until the machinery is under part load to initiate catalytic burning.
The conventional catalytic gas turbine combustor generally employs a pilot zone disposed upstream of the catalyst. This arrangement has two principal disadvantages. First, the catalyst bed is subjected to thermal shock when the pilot zone is ignited. Secondly, when the catalytic operation is initiated, the pilot burner must be extinguished. One method of extinguishing the pilot burner is to interrupt the fuel flow to the pilot. An alternative method is to reduce the fuel supply to the pilot zone while adding fuel to the catalytic main stage. In these arrangements, the fuel air and combustion products from the pilot zone mix and enter the catalytic section of the combustor where the fuel burns. In these instances, there is a danger that autoignition will occur upstream of the combustor which will thermally shock the combustor, increase the pressure drop and unbalance the chamber to chamber mass flow and require that the system be shut down or revert to pilot operation only. Another disadvantage of this type of system is that the face velocity is higher when the pilot zone is fired since the temperature is elevated upstream of the catalyst bed.
The conventional catalytic gas turbine combustors are generally known as fuel staged combustors because they require two separate fuel control systems which can be either active or passive. The operating range of the catalytic reactor can also be extended by adding air staging, commonly called variable geometry in the art. In an air staged system, less air is introduced upstream of the catalyst when the combustor fuel flow requirement is low, i.e., the combustor air flow distribution is varied so as to maintain the catalyst fuel-air ratio within a narrow range. While variable geometry combustors offer a number of theoretical advantages, they are generally impractical because of the enormous hardware complexity required.
One system which has been proposed to burn any unspent fuel in the catalyst bed effluent is to provide a secondary thermal combustion chamber downstream of the catalyst bed. One such arrangement is described in Flannigan, U.S. Pat. No. 4,047,877 in which a thermal burner which is disposed for directing jets of burning gaseous fuel such as natural gas from a multiplicity of points just downstream of the catalyst is described. A fuel-air mixture is ignited in a pilot zone during start-up and passed through the catalyst zone into the secondary combustion chamber. The secondary chamber then burns any unspent fuel in the catalyst bed effluent.
It is accordingly the object of this invention to provide a new catalytic combustor and method of operation which maintains the desirable features of catalytic combustors, particularly low NOx emissions, while accomplishing gas turbine ignition and acceleration without the use of catalytic combustion and without combustion upstream of the catalytic combustor thereby decreasing the potential for imposing a thermal shock on the catalyst bed. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description in which the sole FIGURE is a schematic representation of a catalytic combustor constructed in accordance with the invention.